A printer, a copying machine, and a multi-functional peripheral (MFP) using an electrophotographic process are known. Two types of means called laser optical system (laser scan unit (LSU)) and print head (imaging bar) are known as light exposure means (light exposure unit) for those devices. The laser optical system exposes a photosensitive drum to light by reflecting a laser beam from a semiconductor laser element via a rotatable polygon mirror and scanning the photosensitive drum. The print head exposes a photosensitive drum to light by scanning the photosensitive drum with light emitted from a plurality of light emitting elements such as light emitting diodes (LEDs).
The laser optical system consumes a lot of power and generates operation noise in forming an image because the laser optical system needs to rotate the polygon mirror at high speed. Moreover, the laser optical system has a large size in many cases because the laser optical system needs a mechanism that scans laser light and a lens group for forming an image of scan light on the photosensitive drum.
On the other hand, the print head can achieve downsizing with a structure in which an image of light emitted from a plurality of light emitting elements is formed on the photosensitive drum via small-size lenses that form an erect image, which is called rod lens array. Further, the print head is a less-power consumption, silent light exposure unit because the print head does not include a movable portion.
Some of developed print heads use organic light emitting diodes (OLEDs) instead of the LEDs (array of LED chips).
The print head using the LEDs typically has LED chips arrayed on a printed board.
The print head using the OLEDs has the OLEDs formed on a substrate by using a mask at a time. In the print head using the OLEDs, light emitting elements can be precisely arranged. For example, a print head in which a plurality of light emitting elements including the OLEDs are formed on a glass substrate is known.
The plurality of light emitting elements of the print head corresponds to one line in a main scanning direction. Each of the light emitting elements emits light on the basis of pixel information read from a page memory. Further, various technologies have been proposed, concerning how to read the pixel information stored in the page memory for image tilt correction.